Guard cell sizes and ploidy levels in Polylepis (Rosaceae)

ABSTRACT

The Andean tree genus Polylepis (Rosaceae) has recently been recognized to include polyploid species, but their occurrence within the genus is still incompletely known, especially in light of a forthcoming taxonomic treatment based on a narrow species concept including morphological, climatic and biogeographic distinctness that recognizes 45 species. We obtained guard cell measurements as proxies of ploidy level from 114 individuals of 33 species of Polylepis, including all species for which no previous measurements were available. In combination with previously published data, also on nucleus mass and chromosome counts, we infer that on current knowledge 19 (42%) species are probably purely diploid, 15 (33%) purely tetraploid, and one (2%) purely octoploid. The remaining eight (18%) species have mixed ploidy levels, with three (7%) being di- and tetraploid, two (4%) di- and hexaploid, and one each tetra- and hexaploid, tetra- and octoploid, and di-, tri-, tetra- and hexaploid. Based on our understanding of the evolutionary relationships in Polylepis, it would appear that polyploidy has originated at least about eight times independently in the genus, sometimes as autopolyploidy, sometimes as a result of interspecific hybridization, and sometimes in relation to cultivation. The taxonomic implications of the ploidy levels are complex, in some cases supporting species-level distinction and in others posing the question whether different ploidy levels within a species should better be treated as distinct species. Ploidy level needs to be taken into account for the conservation of the genus, as for example if different populations of a species have different ploidy levels, mixing these origins in reforestation schemes may lead to the formation of sterile hybrids. Guard cell measurement is a low cost and simple technique that can be readily used on both live and dried plant material for such applications, but it has limitations and further data on chromosome counts and nucleus mass are also needed to fully understand the evolution of ploidy levels in Polylepis and its implications.

KEYWORDS:

• Evolution
• polyploidy
• taxonomic relevance

Acknowledgments

We thank the curator of the herbaria (AAU, COL, CUZ, F, GOET, HUA, LOJA, MEDEL, MERF, MO, NY, QCA, US, USM, VEN, and Z/ZH) for loans of specimens, M.C. Segovia S. for allowing us to include unpublished data from her PhD thesis, Mario Coiro for sharing the chemical preparation method. For support of the present study, we thank FONDECYTCONCYTEC (No227-2014-FONDECYT) for providing funding to TEBE, and to the Servicio Nacional Forestal y de Fauna Silvestre-Peru, for providing research authorizations under the R.D.G. No 233-2015, No 237-2015-SERFOR/DGGSPFFS, and Ministerio del Ambiente-Ecuador for providing research authorization under MAE-DNB-CM-2018-0082. T. Dickinson and an anonymous reviewer provided critical comments that greatly improved the manuscript.

Author contribution

This work is part of the Ph.D. thesis research of T. E. Boza E. and was developed under the supervision of M. Kessler. Both authors consolidated the ploidy database and contributed to any taxonomic decision adopted in this paper. The labwork was done by V. Popp. All authors provided comments on drafts of this manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by FONDECYT-CONCYTEC [227-2014-FONDECYT] FONDECYT stands for  Fondo Nacional de Desarrollo Científico, Tecnológico y de Innovación Tecnológica and CONCYTEC stands for Consejo Nacional de Ciencia, Tecnología e Innovación Tecnológica both from Perú.